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/* RFC2631.java --

   Copyright (C) 2003, 2006 Free Software Foundation, Inc.

This file is a part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2 of the License, or (at

your option) any later version.

GNU Classpath is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

You should have received a copy of the GNU General Public License

along with GNU Classpath; if not, write to the Free Software

Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301

USA

Linking this library statically or dynamically with other modules is

making a combined work based on this library.  Thus, the terms and

conditions of the GNU General Public License cover the whole

combination.

As a special exception, the copyright holders of this library give you

permission to link this library with independent modules to produce an

executable, regardless of the license terms of these independent

modules, and to copy and distribute the resulting executable under

terms of your choice, provided that you also meet, for each linked

independent module, the terms and conditions of the license of that

module.  An independent module is a module which is not derived from

or based on this library.  If you modify this library, you may extend

this exception to your version of the library, but you are not

obligated to do so.  If you do not wish to do so, delete this

exception statement from your version.  */

package gnu.javax.crypto.key.dh;

import gnu.java.security.hash.Sha160;

import gnu.java.security.util.PRNG;

import java.math.BigInteger;

import java.security.SecureRandom;

/**

 * An implementation of the Diffie-Hellman parameter generation as defined in

 * RFC-2631.

 * <p>

 * Reference:

 * <ol>

 * <li><a href="http://www.ietf.org/rfc/rfc2631.txt">Diffie-Hellman Key

 * Agreement Method</a><br>

 * Eric Rescorla.</li>

 * </ol>

 */

public class RFC2631

{

  public static final int DH_PARAMS_SEED = 0;

  public static final int DH_PARAMS_COUNTER = 1;

  public static final int DH_PARAMS_Q = 2;

  public static final int DH_PARAMS_P = 3;

  public static final int DH_PARAMS_J = 4;

  public static final int DH_PARAMS_G = 5;

  private static final BigInteger TWO = BigInteger.valueOf(2L);

  /** The SHA instance to use. */

  private Sha160 sha = new Sha160();

  /** Length of private modulus and of q. */

  private int m;

  /** Length of public modulus p. */

  private int L;

  /** The optional {@link SecureRandom} instance to use. */

  private SecureRandom rnd = null;

  /** Our default source of randomness. */

  private PRNG prng = null;

  public RFC2631(int m, int L, SecureRandom rnd)

  {

    super();

    this.m = m;

    this.L = L;

    this.rnd = rnd;

  }

  public BigInteger[] generateParameters()

  {

    int i, j, counter;

    byte[] u1, u2, v;

    byte[] seedBytes = new byte[m / 8];

    BigInteger SEED, U, q, R, V, W, X, p, g;

    // start by genrating p and q, where q is of length m and p is of length L

    // 1. Set m' = m/160 where / represents integer division with rounding

    //    upwards. I.e. 200/160 = 2.

    int m_ = (m + 159) / 160;

    // 2. Set L'=  L/160

    int L_ = (L + 159) / 160;

    // 3. Set N'=  L/1024

    int N_ = (L + 1023) / 1024;

    algorithm: while (true)

      {

        step4: while (true)

          {

            // 4. Select an arbitrary bit string SEED such that length of

            //    SEED >= m

            nextRandomBytes(seedBytes);

            SEED = new BigInteger(1, seedBytes).setBit(m - 1).setBit(0);

            // 5. Set U = 0

            U = BigInteger.ZERO;

            // 6. For i = 0 to m' - 1

            //    U = U + (SHA1[SEED + i] XOR SHA1[(SEED + m' + i)) * 2^(160 * i)

            //    Note that for m=160, this reduces to the algorithm of FIPS-186

            //    U = SHA1[SEED] XOR SHA1[(SEED+1) mod 2^160 ].

            for (i = 0; i < m_; i++)

              {

                u1 = SEED.add(BigInteger.valueOf(i)).toByteArray();

                u2 = SEED.add(BigInteger.valueOf(m_ + i)).toByteArray();

                sha.update(u1, 0, u1.length);

                u1 = sha.digest();

                sha.update(u2, 0, u2.length);

                u2 = sha.digest();

                for (j = 0; j < u1.length; j++)

                  u1[j] ^= u2[j];

                U = U.add(new BigInteger(1, u1).multiply(TWO.pow(160 * i)));

              }

            // 5. Form q from U by computing U mod (2^m) and setting the most

            //    significant bit (the 2^(m-1) bit) and the least significant

            //    bit to 1. In terms of boolean operations, q = U OR 2^(m-1) OR

            //    1. Note that 2^(m-1) < q < 2^m

            q = U.setBit(m - 1).setBit(0);

            // 6. Use a robust primality algorithm to test whether q is prime.

            // 7. If q is not prime then go to 4.

            if (q.isProbablePrime(80))

              break step4;

          }

        // 8. Let counter = 0

        counter = 0;

        while (true)

          {

            // 9. Set R = seed + 2*m' + (L' * counter)

            R = SEED

                .add(BigInteger.valueOf(2 * m_))

                .add(BigInteger.valueOf(L_ * counter));

            // 10. Set V = 0

            V = BigInteger.ZERO;

            // 12. For i = 0 to L'-1 do: V = V + SHA1(R + i) * 2^(160 * i)

            for (i = 0; i < L_; i++)

              {

                v = R.toByteArray();

                sha.update(v, 0, v.length);

                v = sha.digest();

                V = V.add(new BigInteger(1, v).multiply(TWO.pow(160 * i)));

              }

            // 13. Set W = V mod 2^L

            W = V.mod(TWO.pow(L));

            // 14. Set X = W OR 2^(L-1)

            //     Note that 0 <= W < 2^(L-1) and hence X >= 2^(L-1)

            X = W.setBit(L - 1);

            // 15. Set p = X - (X mod (2*q)) + 1

            p = X.add(BigInteger.ONE).subtract(X.mod(TWO.multiply(q)));

            // 16. If p > 2^(L-1) use a robust primality test to test whether p

            //     is prime. Else go to 18.

            // 17. If p is prime output p, q, seed, counter and stop.

            if (p.isProbablePrime(80))

              {

                break algorithm;

              }

            // 18. Set counter = counter + 1

            counter++;

            // 19. If counter < (4096 * N) then go to 8.

            // 20. Output "failure"

            if (counter >= 4096 * N_)

              continue algorithm;

          }

      }

    // compute g. from FIPS-186, Appendix 4:

    // 1. Generate p and q as specified in Appendix 2.

    // 2. Let e = (p - 1) / q

    BigInteger e = p.subtract(BigInteger.ONE).divide(q);

    BigInteger h = TWO;

    BigInteger p_minus_1 = p.subtract(BigInteger.ONE);

    g = TWO;

    // 3. Set h = any integer, where 1 < h < p - 1 and h differs from any

    //    value previously tried

    for (; h.compareTo(p_minus_1) < 0; h = h.add(BigInteger.ONE))

      {

        // 4. Set g = h**e mod p

        g = h.modPow(e, p);

        // 5. If g = 1, go to step 3

        if (! g.equals(BigInteger.ONE))

          break;

      }

    return new BigInteger[] { SEED, BigInteger.valueOf(counter), q, p, e, g };

  }

  /**

   * Fills the designated byte array with random data.

   *

   * @param buffer the byte array to fill with random data.

   */

  private void nextRandomBytes(byte[] buffer)

  {

    if (rnd != null)

      rnd.nextBytes(buffer);

    else

      getDefaultPRNG().nextBytes(buffer);

  }

  private PRNG getDefaultPRNG()

  {

    if (prng == null)

      prng = PRNG.getInstance();

    return prng;

  }

}